1. Field of the Invention
The present invention relates to the use of oxidizing reactive solids for the oxidation of hydrogen sulfide gas and the removal of hydrogen sulfide and other gasses such as carbon dioxide from gas streams containing one or more of the following: hydrogen sulfide, oxygen, carbon dioxide, carbon monoxide, sulfur, hydrogen, water vapor, methane and other hydrocarbon gasses.
2. Description of the Prior Art
Removal of oxidizable impurities such as hydrogen sulfide alone or in the presence of other gasses in the absence or in the presence of high temperature oxidation catalyst has been the subject of extensive research because of the desirability of purifying such mixed gas streams for environmental and other reasons.
As discussed by Gamson and Elkins [Chem. Eng. Prog. 49, 203-15 (1953)], a widely used commercial process for removal of hydrogen sulfide from gas streams is the Claus process. The original Claus process has evolved through several modifications and improvements, but the basic chemical reactions remain unchanged. The overall result of the Claus process is the conversion of hydrogen sulfide to sulfur: EQU H.sub.2 S+1/2O.sub.2 .fwdarw.H.sub.2 O+Sulfur(v) (1)
As noted in Gamson, et al., Chem. Eng. Prog. 49, 203 (1953), the term "Sulfur (v)", hereinafter referred to as simply "S (v)", signifies complex equilibria existing between the elemental forms of sulfur in the vapor state.
In practice Reaction (1) is actually a summation of two reactions. In the first reaction, one-third of the hydrogen sulfide is oxidized to sulfur dioxide in an uncatalyzed gas-phase combustion: EQU 1/3H.sub.2 S+1/2O.sub.2 .fwdarw.1/3H.sub.2 O+1/3SO.sub.2 ( 2)
The products of Reaction (2), along with the remaining two-thirds of the hydrogen sulfide, are reacted over a bed of bauxite or alumina catalyst at about 200.degree. C. to 400.degree. C.: EQU 2/3H.sub.2 S+1/3SO.sub.2 .fwdarw.2/3H.sub.2 O+S(v) (3)
The bed must be maintained at a sufficiently high temperature in order to prevent condensation of sulfur because difficulties, such as catalyst poisioning, are encountered in the Claus process at low temperature. On the other hand, because the reaction becomes thermodynamically more favorable at a lower temperature, it is desirable to carry out Reaction (3) at as low a temperature as is feasible.
Attempts have been made to carry out direct oxidation of hydrogen sulfide to sulfur by Reaction (1), thereby avoiding the thermodynamic limitations imposed by Reaction (3). An example of this process is disclosed by Baehr, et. al. (U.S. Pat. No. 2,200,529). That patent purports to disclose a converson of a large fraction of hydrogen sulfide directly to sulfur in a free flame in the presence of the stoichiometric amount of oxygen.
Gamson (U.S. Pat. No. 2,594,149) noted that Reaction (1) becomes the principal reaction at very high temperatures. That patent discloses a process for converting hydrogen sulfide directly to sulfur in yields of about 75% at temperatures above about 1300.degree. C., followed by a rapid cooling of the product gasses to minimize formation of sulfur dioxide by reaction of residual oxygen and sulfur product. Gamson also discloses that the oxidation of hydrogen sulfide does not appear to proceed at a practical rate at temperatures below about 500.degree. C. in the absence of catalysts. The patent to Gamson does not disclose a catalytic, low temperature conversion of the type disclosed in the present invention.
An article by Grekel, Oil and Gas Journal Vol. 57, 76-79 (1959), discloses a method for the direct oxidation of hydrogen sulfide to sulfur in the presence of bauxite at temperatures between about 200.degree. C. and about 800.degree. C. Grekel states that, at low space velocities, lower temperatures are preferred, inferring that Reaction (3) is involved. Grekel further implies that at higher space velocities, Reaction (3) becomes less important.
The patent to Thompson (U.S. Pat. No. 1,922,872) discloses a method of oxidizing hydrogen sulfide to sulfur in the presence of oxygen over a bauxite catalyst in the temperature range from about 225.degree. C. to about 275.degree. C. The Thompson catalyst contained about 60% alumina as well as small percentages of ferric oxide, titanium dioxide, and silica. The preferred low temperatures of the Thompson disclosure would seem to suggest that Reaction (3) is controlling. However, the catalyst at the reported temperatures displays a tendency to collect sulfur vapors which poison the catalyst.
Hass (U.S. Pat. No. 4,171,347) discloses a method for the conversion of hydrogen sulfide directly to sulfur in the presence of oxygen and a vanadium oxide catalyst on a non-alkaline support at temperatures from about 120.degree. C. to about 230.degree. C. The vanadium oxide catalyst appears to convert a portion of the hydrogen sulfide to sulfur dioxide. The sulfur dioxide product seems to react with unreacted hydrogen sulfide, for example by Reaction (3), on the same catalyst bed, to form sulfur.
The patent issued to Scott (U.S. Pat. No. 4,164,544) discloses a cyclic process for desulfurizing hot reducing gas by contacting the gas with a desulfurizing agent, regenerating the spent desulfurizing agent, and then reusing the regenerated desulfurizing agent. As disclosed in Scott, a bed of sintered porous pellets comprising the reaction product of manganese oxide and aluminum oxide as the desulfurizing agent is used. Regeneration of the desulfurizing agent is performed by contacting the bed of pellets with an oxidizing gaseous atmosphere. The Scott process appears to result in the formation of only SO.sub.2, in contrast with the present invention in which little or no SO.sub.2 is formed depending on the mode of operation. In Scott, the temperature of the bed is maintained between about 500.degree. C. and about 1300.degree. C. in both the desulfurization and regeneration step. Although Scott uses the term "reaction product" to describe the porous pellets, this term apparently is not meant to indicate a chemical reaction between manganese oxide and aluminum oxide; rather, the term appears intended to describe the product produced by intimately mixing manganese oxide with aluminum oxide, forming the mixture into a pellet, and sintering, by heating the pellet.
In the "iron-sponge" process, as discussed by Goar, Oil and Gas J. 84, 86 (1971), hydrogen sulfide is reacted with ferric oxide to form ferric sulfide. Although the bed of ferric sulfide appears to be able to be reconverted by air oxidation to ferric oxide, the bed eventually becomes saturated with sulfur solids and must be replaced. The iron-sponge process is usually limited to treating gasses containing low concentrations of hydrogen sulfide, due to the economics of bed replacement. Furthermore, the sulfur removed by this process is usually lost because the "spent" bed of iron oxide is usually discarded.
Zinc oxide appears to be frequently used to remove hydrogen sulfide from mixed gas streams containing low concentrations of hydrogen sulfide. ("Ammonia" Part II, A. V. Slack and G. R. James, Marcel Dekker, Inc., New York, 1974, p. 338). This application resembles that of the iron-sponge process; the zinc sulfide product cannot be readily regenerated and is usually discarded after a single application.
The patent to Weiss (U.S. Pat. No. 1,971,168) discloses the preparation of oxidation catalysts represented by the general formula M(HMnO.sub.3).sub.2 and M(MnO.sub.3) wherein the symbol M represents one of various possible metals capable of forming the catalyst in association with manganese. These metals, termed acid manganites, were shown to be useful in the oxidation of carbon monoxide to carbon dioxides. The Weiss patent does not disclose removal of any impurity other than carbon monoxide from a mixed gas stream.
The U.S. patent issued to Haacke (U.S. Pat. No. 3,914,389) discloses a compound having the empirical formula LaCu.sub.0.5 Mn.sub.0.5 O.sub.3 as an oxidation catalyst to convert carbon monoxide to carbon dioxide in automobile exhaust systems.
The patent issued to Remeika et al. (U.S. Pat. No. 4,001,371) discloses a catalyst of the formula La.sub.0.8 K.sub.0.2 MnO.sub.3 to catalyze the conversion of NO.sub.x pollutants to materials N.sub.2 O, N.sub.2, and oxygen. This catalyst is also said to be effective for promoting reactions such as the oxidation of carbon monoxide to carbon dioxide. Each of the catalysts disclosed in the Remeika patent requires the presence of at least one rare earth element, i.e., La, Pr, and Nd, as a component.
None of the above descriptions or disclosures disclose the methods of the present invention in which a reactive solid is used for removing impurities such as hydrogen sulfide and carbon dioxide from gas mixtures.